Field of the Invention
Embodiments of the invention generally relate to photovoltaic devices, such as solar cells, and methods for fabricating such photovoltaic devices.
Description of the Related Art
As fossil fuels are being depleted at ever-increasing rates, the need for alternative energy sources is becoming more and more apparent. Energy derived from wind, from the sun, and from flowing water offer renewable, environment-friendly alternatives to fossil fuels, such as coal, oil, and natural gas. Being readily available almost anywhere on Earth, solar energy is becoming a viable alternative.
To harness energy from the sun, the junction of a solar cell absorbs photons to produce electron-hole pairs, which are separated by the internal electric field of the junction to generate a voltage, thereby converting light energy to electric energy. The generated voltage can be increased by connecting solar cells in series, and the current may be increased by connecting solar cells in parallel. Solar cells may be grouped together in modules on solar panels. An inverter may be coupled to several solar panels to convert DC power to AC power.
One issue with the use of photovoltaic devices such as solar panels is the problem of shading on portions of the solar panel. As shown in FIG. 1, solar panels include multiple photovoltaic cells 10 connected in series in a string in each module to provide increased power and voltage from sunlight. However, a portion of these cells may get shaded from sunlight during operation, which affects the performance of the entire string or module. For example, the cell 12 is shaded from sunlight by an obstruction while the other cells 10 are not. A series mismatch occurs when the electrical parameters of one solar cell are significantly altered from those of the other cells. Since the current through the cells must be the same, the overall current from the combination cannot exceed that of the shaded cell. At low voltages, when one solar cell is shaded while the remainder in the string or module are not, the current being generated by the unshaded solar cells may be dissipated in the shaded cell rather than powering the load. Thus, in a series connected configuration with current mismatch, severe power reductions can occur if the poor cell produces less current. If the configuration is operated at short circuit or low voltages, the highly localized power dissipation in the shaded cell can cause local “hot spot” heating, avalanche breakdown, and irreversible damage to one or more of the solar cells and to the module.
One solution to the effects of mismatch from shading on some solar cells is to use one or more bypass diodes. Solar cells that intrinsically have a very high breakdown voltage or low shunt resistance may not need bypass diodes, but many other types including high performance solar cells such as gallium arsenide (GaAs) solar cells, may need the bypass function. For example, as shown in FIG. 2, typically one or more bypass diodes 14 are connected in parallel and with opposite polarity to solar cell circuits 16. To reduce costs, a bypass diode is usually placed across a group of solar cells. In normal (unshaded) operation, each solar cell 16a is forward biased and the bypass diode 14a is reverse biased and is an open circuit. If one or more of the solar cells 16b become shaded, those cells 16b are reverse biased due to the mismatch in short-circuit current between series connected cells, and the bypass diode 14b is forward biased and conducts current, which allows the current from the unshaded solar cells to flow in an external circuit rather than forward biasing each unshaded cell. The maximum reverse bias across the shaded cell is reduced to about a single diode drop, limiting the current and preventing hot-spot heating and damage to the solar cells.
Although bypass diodes are effective in reducing the destructive effects of mismatches in solar cells due to shading, they are an additional component that must be fabricated for solar cells and thus add to the cost and time of production of solar panels. Furthermore, the bypass diodes must be integrated into a solar cell design, which may be complex and difficult to accomplish. These factors add to the currently higher cost of producing solar cells, may reduce the capability of solar cells in becoming a mainstream energy source, and may limit the applications to which solar cells may be suited.
Accordingly, there is a need for increased efficiency and production compatible methods for providing bypass diode functionality in photovoltaic devices.